"The poor birth outcomes reported include those that have been detected in infants infected with Zika before or during birth, including microcephaly, calcium deposits in the brain indicating possible brain damage, excess fluid in the brain cavities and surrounding the brain, absent or poorly formed brain structures, abnormal eye development, or other problems resulting from damage to brain that affects nerves, muscles and bones, such as clubfoot or inflexible joints."

The women in the U.S. with Zika contracted the virus while traveling, according to the CDC. But the agency also warns of localized Zika outbreaks in the states once mosquito season begins.

So far, 234 pregnant women have been diagnosed with Zika in the U.S., and 189 more women (not counting the six Zika cases reported in the U.S. on Thursday) have been infected in U.S. territories, predominantly Puerto Rico, according to the CDC.